The present disclosure relates to electronic control valves; specifically, to a noise mitigation device for an electronic control valve; more specifically, to such a device acting as a dynamic absorber of valve train motion.
An electronic control valve (eCV) generates an undesirable audible noise over a given supply current range. This noise is caused by the moving portion of the valve, i.e., the valve train, repetitively impacting a stationary portion of the valve, e.g., the valve seat.
An eCV may be used, for example, in an automotive air conditioning system to control compressor capacity by regulating gas entering the compressor crankcase. A prior such eCV is shown in FIG. 1. Higher eCV supply current increases compressor capacity and lower eCV supply current decreases compressor capacity. The eCV current can be changed by changing the applied voltage. In the case of today's vehicles, the applied voltage is changed by pulsing battery voltage on and off, typically at 400 Hz. One 400 Hz cycle is 2.5 milliseconds. Thus, a 400 Hz eCV is regulated through pulse width modulation (PWM), i.e., by altering the portion of each 2.5 millisecond period (the PWM cycle) that a voltage pulse is present. Duty cycle is the time fraction of the PWM cycle that the eCV is energized. The duty cycle represents that time fraction of each PWM cycle during which an energizing voltage pulse is present as a percentage, with average voltage and current increasing as a function of an increased duty cycle. At 50% duty cycle, full voltage is applied for 1.25 milliseconds and no voltage is applied for 1.25 milliseconds. Audible eCV noise is greatest near 50% duty cycle because the valve train can be driven at its maximum displacement in both directions. When energized, the eCV is activated and the actuating magnetic field of the valve pushes against the valve train inertia and a return spring. When the actuator is de-energized, the eCV is unactivated and the return spring pushes the valve train back towards its off-mode rest position. At low duty cycle the valve train has less time to accelerate toward the valve seat. At high duty cycle, the valve train has less time to accelerate back towards its rest position. With lower displacement, the valve train velocity is lower and cannot impact the valve seat with such a high contact force.
The eCV must be a very low friction device to function properly. Conceptually, a good eCV design approaches a classic spring mass system which is frictionless and has very low damping. The result is relatively high amplitude valve train cyclic motion excited by the supply voltage frequency, also known as the forcing frequency. At around 50% duty cycle the valve train repetitively strikes the valve seat at the frequency of the supply voltage with high velocity. The repetitive impact creates an audible, undesirable buzzing noise.
This noise is a function of, but not limited to: supply voltage frequency; coil inductance; valve train mass; valve train return spring force; valve train damping; and valve seat geometry. Prior efforts to minimize this valve noise in automotive applications include: changing coil inductance by increasing coil size; changing supply voltage frequency; changing supply voltage waveform characteristics; adding friction by direct contact between the valve train and valve body; suppressing noise transmission through air and structure; changing valve seat impact zone compliance; changing valve train mass; and changing valve train return spring force.
Some such efforts have proven somewhat effective, but have not been successful in satisfactorily resolving the problem for various reasons. Changing coil inductance by increasing coil size is effective, but increases cost and packaging. Changing supply voltage frequency or waveform characteristics is effective, but is not acceptable to most customers. Adding friction by direct contact between the valve train and valve body is effective, but negatively impacts valve performance and has proven unpredictable during valve operation. Suppressing noise transmission through air and structure is partially effective, but increases cost and negatively impacts durability and packaging. Changing valve seat impact zone compliance is effective, but negatively impacts performance and durability. Changing valve train mass or return spring force has not been effective within reasonable limits.
In practice, prior attempts to solve this problem by eCV suppliers have primarily involved adding direct contact friction between the valve train and valve body via various methods such as: adding grease between the valve train and valve body; utilizing an O-ring to provide friction between valve train and valve body; and configuring the actuating magnetic field to be non-symmetrical, whereby the field induces a friction-generating side load between valve train and valve body. As noted above, however, introducing friction directly between the valve train and the valve body impairs valve performance and introduces performance unpredictably.
A noise mitigation device that overcomes the shortcomings of the above-discussed prior efforts would be a desirable advancement in the relevant art.